Thymic-derived T regulatory cells (Treg) express the lineage commitment transcription factor Foxp3 and play critical roles in all aspects of immune responses including control of the interaction of the host with microbes. The mechanisms by which Tregs modulate responses to infectious agents are complex and include limiting exuberant inflammatory responses with control of tissue damage, facilitation of the persistence of the pathogen for maintenance of immunity, but also enhanced pathogen replication and suppression of effector T cell function. Progress in this area has been limited because of our lack of knowledge regarding the nature of the antigens recognized by Tregs. The mouse model of infection with lymphocytic choriomeningitis virus (LCMV) allows one to differentiate between the effects of acute and chronic viral infection on Treg activation. Our initial approach to exploring the role of Treg during LCMV infection was to compare the frequency and phenotype of Treg in C57BL/6 mice during the course of Armstrong and Clone 13 LCMV infection. Treg dramatically increased in frequency among splenic CD4+CD8- T cells during Clone 13, but not Armstrong, infection. This increase in the frequency of Foxp3+ Treg was transient and peaked at 17 days post infection (dpi). Treg from Clone 13 infected mice displayed a much more activated phenotype. A marked increase in the frequency of Foxp3+ Treg expressing TCR Vbeta5 was observed, again specifically during Clone 13 infection. There was no change in the percentage of Treg expressing any of the other Vbeta analyzed and no change in the frequency of CD4+Vbeta5+Foxp3- cells was noted during either Armstrong or Clone 13 infections. The failure to observe Treg expansion during Armstrong infection was secondary to the absence of viral chronicity and not to the minor sequence difference between the two strains. Studies in other infectious disease models suggest that Tregs in infected animals may be specific for the infectious organism or perhaps specific for ubiquitous or tissue-specific antigens whose expression is enhanced during the course of the infection. We saw no evidence for Treg cell specific reactivity in response to a library of overlapping peptides that span the entire LCMV proteome. Because the expansion of the Treg in mice chronically infected with LCMV was restricted to a specific Vbeta segment, we also considered the possibility that a superantigen (Sag) may be driving proliferation. As LCMV has not been previously shown to express a superantigen, a likely explanation for our results was that the expansion of the Vbeta5+ Treg was secondary to stimulation by an endogenous Sag such as mouse mammary tumor virus-(Mtv) encoded Sags. To examine this possibility, we infected BALB/c, that normally express Mtv-6, -8, and -9 and delete Vbeta-5, -11, and -12 T cells in the thymus, with Clone 13 and analyzed the effects of infection on the Vbeta repertoire. We observed a significant increase in the percentage of CD4+Foxp3+Vbeta5+ and a dramatic increase in Foxp3+Vbeta12+ Treg, but no change in the percentage of CD4+Foxp3-Vbeta5+ or Foxp3-Vbeta12+ T cells, following Clone 13 infection. To directly prove that expansion of the Vbeta5 and Vbeta12 Foxp3+ subsets in BALB/c mice was secondary to activation by endogenous Mtv-Sag, we infected BALB/c mice lacking all three of the endogenous Mtv proviruses (Mtv-null mice). Consistent with a lack of Mtv-Sag, these mice had a substantial population of T cells that expressed either Vbeta5 or Vbeta12 in Foxp3+ and Foxp3- subsets. However, we observed no change in the frequencies of either Vbeta5+ or Vbeta12+ Treg in these mice following infection, thus demonstrating that the observed Vbeta-specific Treg expansion was, in fact, due to stimulation by an endogenous retroviral superantigen. In collaboration with Dr. P. Pasquali, we have also extended our studies on Treg modulation of the immune response to infectious agents to their role in modulating the immune response to an infection with an intracellular bacteria, B. abortus. After infection with this agent, there is a marked increase of both effector and regulatory T cells in the spleen. Mice lacking Treg were more resistant to infection, produced more interferon-gamma, and had an increased capacity to control the infection. The capacity of Treg cells to control interferon-gamma production may be of great benefit in brucellosis as interferon-gamma is responsible for exaggerated inflammation and immunopathology in this infectious disease model. On the other hand, the ability of Treg cells to downmodulate the immune response may lead to establishment of chronic infection.